BTO used to mean that you asked for X Y Z and some assembly tech in the USA pulls those parts and inserts them into your bare machine. MacPro with X memory, Y storage, Z graphics card, all started from the same assembly line “bare” MacPro waiting for parts. Stock configs were assembled to stock configuration and shipped out to retailers on the assembly line.
Macs are SoC now. Maybe, maybe, they have bins of different boards sitting idle ready to plop in a chasis and ship. But that wouldn’t be efficient.
In reality, Apple makes a BATCH of various configurations and ships them to a drop ship warehouse. When you BTO your Mac, you are simply assigned a unit from the warehouse. If none are available, you are assigned a unit from the next batch, or in the queue for the next. If they underestimated demand for certain configs, next run may change the mix.
None of these BTO machines are sent to bulk retailers. Only “stock” configs are sent to retailers. Specialists may get more options, but not all options.
In the past, running out of specific configs was uncommon unless a graphics card was in limited supply. All BTOs took about the same time to assemble and deliver.
Now, during a mature production cycle, certain configs will ship in a week and others longer because when you order you are just getting a place in line for that specific configuration. Your “order now” click doesn’t connect to China or Vietnam directly and say “build this computer for Charlie.” And because of that, some customers end up buying a different configuration because they need it now, not in the summer.
So it’s not BTO.